Mechanical dock levellers are used very extensively to bridge a height differential between a truck bed and a loading dock. The dock leveller includes a ramp pivotally connected to a support structure. A spring biased device is used to raise the ramp to enable a truck to back up to the dock. The ramp is then lowered onto the truck bed usually by way of a person walking up onto the ramp to overcome the upward spring bias of the raising device. To hold the dock leveller ramp at the height of the truck bed, a hold-down mechanism is used which holds the ramp at the truck bed height against the upward spring biasing force of the raising device. Assuming that the ramp is adjusted for the height of the truck bed when fully loaded, then it is a common occurrence for the truck bed to rise as it is unloaded due to the lightening of the load and the truck springs pushing upwardly on the truck bed. Such upward raising of the truck bed can exert considerable forces on the ramp. Attempts have been made to include provision for allowing the ramp to rise without causing damage to the hold-down mechanism.
As disclosed in U.S. Pat. No. 3,316,575 to Blue Giant Equipment, the hold-down mechanism has a reel for paying out and reeling in a cable, which is connected at its free end to the ramp. The hold-down mechanism is secured to the bottom of the support frame. A knurled wheel is positioned to wedge between the reel circumferential ledge and a wedge portion of the hold-down device. When the knurled wheel is wedged in place, rotation of the reel in a cable pay out direction is prevented. To provide for desired elevation of the ramp, the knurled wheel is moved out of the wedged position by an appropriate release device. To accommodate the raising of the truck bed during unloading, the cable is connected to a spring which provides for a slight degree of ramp elevation without causing damage. However as the truck bed rises, greater force is exerted on the hold-down mechanism as tension in the spring increases. As is appreciated, an appropriate balance between the tension in the spring and the ability to hold the ramp in the desired position without bouncing must be achieved. Furthermore, the spring cannot always accommodate excessive rise in some types of truck bed systems so that excessive forces are exerted on the hold-down mechanism which usually breaks the mechanism. Another problem with the spring system is that the hold-down mechanism does not adjust to the elevated heights of the truck bed. As soon as the truck pulls away from the dock, the extended spring forces the dock leveller ramp to slam downwardly thereby exerting additional forces on the hold-down mechanism which can cause damage to it.
An alternative mechanism is disclosed in Smith et al, U.S. Pat. No. 4,126,909. A resilient biasing of the hold-down is provided to accommodate elevation of the truck bed. A coiled spring permits upward movement of the ramp as a truck is unloaded. A crank is connected to the coiled spring which is swung clockwise away from a stop in opposition to the resilient urging of the torsion spring as the truck bed rises. The coiled spring, as it is wound tighter, acts through the link arm to urge continually the ramp downwardly onto the truck bed. However, the coiled spring arrangement is not readily adjusted as it weakens and suffers from the same problems as the spring system in U.S. Pat. No. 3,316,575. Attempts have been made by Smith et al to avoid the coiled spring arrangement by setting up other spring designs. However, as the springs wear out the effectiveness of the hold-down mechanism deteriorates.
A variety of hold-down mechanisms for dock levellers also incorporate one type or another of ratchet arrangement, such as disclosed in U.S. Pat. No. 4,531,248. To accommodate rising of the truck bed, the hold-down mechanism may be automatically released to permit the ramp to rise. The ratchet system is mounted on a spring loaded piston. As the ramp rises, the spring loaded piston is compressed until a shortened length of chain becomes taut to release the hold-down mechanism. The ramp will then spring up until the spring loaded piston resumes its normal position at which point the chain becomes relaxed and re-engages the ratchet. The problem with this arrangement is undue wear caused in the ratchet teeth due to the constant releasing and re-engaging of the ratchet during unloading of the truck. As the teeth become worn, the system can be dangerous because the teeth do not engage properly resulting in the ramp rising well above the truck bed which can cause an upset of a forklift truck.
To avoid the hold-down mechanism of the mechanical dock leveller interfering with the raising of the truck bed, a system was developed by Zajac et al, U.S. Pat. No. 3,249,956 which allows the ramp to float upwardly free of the ramp raising device and is held in any one position by the hold-down mechanism. The ramp raising device includes two spring loaded arms, one of which can be held at any position by a hold-down mechanism. The hold-down mechanism comprises a friction brake which, when released, allows the one arm to swing upwardly to raise the ramp in conjunction with the other spring loaded arm to its uppermost position. The ramp may be lowered to any desired height and with the hold-down engaged and weight on the ramp, the hold-down will hold the ramp in the desired position. As the truck bed rises, due to unloading of the truck, the ramp will elevate upwardly away from the ramp raising arm having the hold-down device. Hence no modification is required in the hold-down mechanism to compensate for this raising of the ramp away from the ramp raising arm. However, this system requires the use of an elaborate frictional brake design to withstand the loads exerted on the hold-down mechanism by the spring system which elevates the ramp. The system requires the use of two sets of springs with the hold-down functional on only one set of the springs. In this manner, one set of springs does not provide sufficient upward bias to raise the ramp whereas both sets of springs in combination have sufficient upward force to raise the ramp. When the truck pulls away from the dock, the ramp is then free to fall onto the ramp raising mechanism which can cause damage to it.
Layne, U.S. Pat. No. 3,167,796 discloses a ratchet wheel and pawl arrangement for use as the hold-down mechanism in holding the ramp at any desired height. Cables are wound around the ratchet wheel such that as the ramp rises and a first cable is payed out, a second cable is reeled in to act as a return for the ratchet wheel when the ramp moves downwardly to reel in the first cable. The first cable is wound around a spring loaded sheave to accommodate upward movement of the ramp due to elevation of the truck bed. Such upward movement of the ramp avoids damaging the hold-down mechanism. However, the extent of upward movement is limited by the extent of travel of the spring loaded sheave.